1. Field of the Invention
The present invention relates to driving control of a lens attached to a camera.
2. Description of the Prior Art
Conventionally, single-lens reflex (SLR) systems of cameras have been premised on the use of various types of interchangeable lenses, and in-focus photos cannot be obtained unless the lens is stopped at the in-focus position with accuracy in any types of interchangeable lenses in automatic focusing. However, stopping or damping characteristics of interchangeable lenses vary widely, and automatic focusing cannot accurately be performed with a response of the shortest time unless the stop control is performed in consideration of the damping characteristic of each lens.
The stop control can be performed by providing a control parameter associated with the damping characteristic of each type of interchangeable lenses. However, it cannot be said that this is a desirable countermeasure, because it is predicted that various types of interchangeable lenses will be put on the market in the future and it is considered that lens data will increase accordingly. According to a conventional method, of various types of interchangeable lenses, a lens that is poorest in damping characteristic and most disadvantageous in the stop control is selected, a control parameter is decided so that this lens can accurately be stopped in the shortest time, and the control parameter is also applied to other interchangeable lenses.
FIG. 1 is a graph showing the conventional camera lens stop control method. Here, the lateral axis represents the drive time t, and the longitudinal axis represents the number of revolutions rps of the drive motor. As shown in the figure, in the lens driving control, typically, the motor is activated with full energization at a point p.sub.1, and the lens movement speed becomes constant at a point p.sub.2 and starts to be reduced at a point p.sub.3. The point p.sub.3 is the first speed reduction point decided so that the lens that is poorest in damping characteristic and most disadvantageous in the stop control, that is, the lens whose driving amount from the application of the brake to the complete stop is largest is accurately stopped.
Therefore, as shown in the figure, the lens being most disadvantageous in the stop control is slowed at "a" fixed rate as shown by the line A, and at a point a, in-focus condition is obtained in the shortest time for the lens and the lens is stopped. The actual driving amount is represented by the area surrounded by the lines of the graph (in this example, the lines connecting the points p.sub.1, p.sub.2, p.sub.3 and "a") and the lateral axis, that is, the integration value of the graph.
However, according to the conventional lens stop control method, since the first speed reduction point is decided so that the lens being most disadvantageous in the stop control is accurately stopped as mentioned above, when a lens having a different damping characteristic is stop-controlled, for example, the speed reduction of the lens is represented by a curve as shown by the line B of FIG. 1. In this case, since the damping characteristic is comparatively excellent, the time required for the lens movement speed to be reduced to a predetermined speed is shorter accordingly and the driving amount is smaller, so that if the speed reduction is performed as it is, the lens movement speed is reduced as shown by the line C and the lens is stopped at a point "c" before in-focus condition is obtained. Therefore, in order for the lens to be stopped at the in-focus position, the lens is moved at constant speed in the ranges shown by "s" in the figure.
In-focus condition is obtained at point "b" of the figure and the lens is stopped. However, as is apparent from comparison with the point "a", the control time can be longer than that in the case of the lens being most disadvantageous in the stop control. When the stop control is performed by appropriately delaying the speed reduction point to make the most of the damping characteristic intrinsic to this lens, it should be possible to perform the stop control in a shorter time without being affected by the stop control time of the lens being inferior in damping characteristic.
FIG. 2 is a graph showing a stop control method to be carried out. Here, like in FIG. 1, the lateral axis represents the drive time t, and the longitudinal axis represents the number of revolutions rps of the drive motor. As shown in the figure, when at a speed reduction point p.sub.4 delayed from the speed reduction point p.sub.3 of the lens being most disadvantageous in the stop control, the speed reduction control of a corresponding lens being comparatively excellent in damping characteristic is started, the lens is slowed at a fixed rate as shown by the broken line D, and at a point "d", in-focus condition is obtained in the shortest time for the lens and the lens is stopped. At this time, the time from the activation to the stop is t1 represented by a dimension line, and compared to the time t2 (also represented by a dimension line) from the activation to the stop by the above-described conventional stop control method, the control time decreases by t2-t1.
Thus, to set the most suitable speed reduction point, the stop control can be performed by providing a control parameter associated with the damping characteristic of each type of interchangeable lenses as mentioned in the prior art. However, since there are cases where the same type of interchangeable lenses have different damping characteristics because of nonuniformness among the interchangeable lenses, it cannot be said that this method is a good solution. Further, it is necessary to increase the memory capacity of the camera in accordance with the increase in data due to increase in types of interchangeable lenses.
This problem can be solved by measuring the actual damping characteristic when each interchangeable lens is attached to the camera, and capitalizing on the data to perform the stop control. However, to determine that the lens is actually stopped after the application of the brake in measuring the damping characteristic, it is necessary to wait for a predetermined time to make sure that the lens is not moving. This increases the control time.